An entire Delaware family fell seriously ill while on a Caribbean vacation, and a dangerous chemical allegedly used at their hotel may be to blame.

What happened:

In March, school administrator Steve Esmond, his wife Theresa Devine and their two teenage sons may have been exposed to the pesticide methyl bromide at the Sirenusa Resort on St. John in the U.S. Virgin Islands, according to a report on ABC station WPVI-TV.

The day the family arrived at their second floor condo, the apartment below them was sprayed with methyl bromide to "deal with an indoor bug," according to Judith Enck, Administrator for the Environmental Protection Agency's Region 2 Office, which covers the U.S. Virgin Islands.

The pesticide was applied during the day of March 19, Enck said, and by that night, the entire family "started having adverse health effects." Both boys had seizures, according to Enck.

Paramedics responded and took the family to a hospital on neighboring island St. Thomas. The boys were then airlifted to a hospital in Philadelphia, Enck said, while the parents were airlifted to a hospital in Delaware.

According to the EPA, methyl bromide exposure can have short-term and long-term effects include severe lung injuries and neurological impairment.

The EPA banned methyl bromide for indoor residential use in 1984, Enck told ABC News today, but the product is still on the market for agricultural use.

The family:

Steve Esmond and his sons remain in very critical condition, Enck said today, adding that Devine is not in critical condition.

Steve Esmond is the head of the middle school at the Tatnall School in Wilmington, Delaware, according to the school's website. Theresa Devine is a dentist in Broomall, Pennsylvania, according to the company website.

"They're just one of those families that everyone loves to be around," Oliver Campbell, a peer of one of the boys, told WPVI-TV. "It's just horrible."

"It's terrifying," another peer, Carl Marvin, said to WPVI-TV. "It's really scary to think that this could happen to somebody that you know."

The investigation:

The EPA has launched a "comprehensive investigation," Enck said. Officials were sent to sample and monitor the apartments to see if any of the pesticide was left.

"We're looking at what happened here, which we consider an illegal application of methyl bromide," Enck said.

The EPA also issued a pesticide warning in the Caribbean and is examining if methyl bromide was used in other locations in the U.S. Virgin Islands.

The Department of Justice is investigating Terminix, the company that applied the pesticide, Enck confirmed.

Terminix has halted all fumigation in the Virgin Islands as part of the ongoing investigation, said Peter Tosches, Terminix's Senior Vice President of Corporate Communications.

"First and foremost, the family is in our thoughts and prayers," Terminix said in a statement. "We're cooperating with authorities in their investigation, and we're conducting our own thorough investigation in the matter. We're committed to performing all work we undertake in a way that is safe for our employees, customers and the public."

Sea Glass Vacations, which acts as a rental agent for several rental units at Sirenusa, said in a statement, "Our deepest sympathies go out to the family and all of those impacted by this accident. Our thoughts and prayers hope for a full and speedy recovery."

"We are also aware that authorities are actively investigating the possibility that the illnesses were due to chemicals used for pest control. The unit immediately below Villa Capri was recently treated for pests by Terminix, however, Villa Capri itself had not been so treated," the statement from Sea Glass Vacations added. "Sea Glass Vacations does not treat the units it manages for pests but instead relies on licensed professionals for pest control services. We are committed to full cooperation with all the authorities currently investigating this matter."